The present invention relates to cordless telephones, and in particular, to an out-of-range warning system for said cordless telephones wherein the strength of the carrier signal received by the base station from the handset is monitored to determine whether or not the handset is within operational range. According to aspects of the present invention, if it is determined that the handset is out-of-range, a plurality of warning signals at the determined intervals is transmitted from the base station to the handset as a warning of impending system inoperability. After a predetermined number of warnings, the telephone unit is switched "on" hook and disconnected from the telephone network.
One method previously used was for the handset to generate a guard tone at a frequency above or below that of the transmitted speech, e.g., 40 Hz or 6 KHz, and transmitted from the handset to the base unit station within the audio band used for voice. A sensing system within the base station would periodically test for presence of such a guard tone and if it was determined that the guard tone was no longer present, the assumption was made that the handset was out of operational range from the base station, and the telephone unit would be abruptly disconnected from the telephone network by an "on" hook command. Such an abrupt and unwarned of disconnect can be very unnerving to a user particularly if a telephone conversation is of great importance. Another approach taken in the prior art is to sense the carrier level received by the base station and if such carrier level received from the handset is below a predetermined threshold level, there is a hang up of the line without any warning. Still another approach taken in the prior art is to provide a warning, such as a double beep, if the carrier level received by the base station from the handset is below a predetermined threshold level. However, in such a case, once the warning is issued, there would be no disconnect, so that the user, either not being within operational range of the base station or perhaps with a battery operating the handset which did not retain sufficient charge to operate the handset, would continue using the non-operable handset with the result that the line would be tied up for an indefinite time while the telephone unit was in a non-operational mode.
The present invention overcomes these undesirable operational conditions. According to aspects of the present invention the base unit measures the RF signal level received from the handset and provides a warning tone at the earpiece of the portable handset if the RF signal level is below a minimum operating threshold level. This warning signal is provided such that even if the handset is beyond the normal operating range of the cordless telephone, the user can still hear the signal. The base unit will transmit the "out-of-range" signal a predetermined number of times and then, if the portable handset unit does not come back into the operating range of the base unit, the base will then disconnect from the telephone line.